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11th SAGA Biennial Technical Meeting and Exhibition
- Conference date: 16 Sep 2009 - 18 Sep 2009
- Location: Swaziland, South Africa
- Published: 16 September 2009
61 - 80 of 157 results
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The Influence of Curved and Angled Survey Lines on 2D Resistivity Surveys Employing the Wenner (α) Geometr
More LessMulti-electrode resistivity systems, such as the Lund Imaging System, employ protocols that control switching between the various electrode pairs. The standard protocols used for 2D surveying assume that the survey lines are straight so that the various electrode pairs are collinear. However, due to the presence of infrastructure and other surface constraints it is not always possible to conduct 2D resistivity surveys along straight lines. When survey lines are curved/angled, the apparent resistivity values and assumed pseudo-depths are affected by the non-collinearity of the electrode pairs. In addition, the recorded apparent resistivity data may be representative of the subsurface conditions at positions laterally displaced from the survey line. For a curved survey line spanning an angle of less than π/2 radians, the errors in both the calculated apparent resistivities and assumed pseudo-depths are less than 3%. Similarly, for angled survey lines with angles smaller than π/4 radians, these errors remain below 6.5% and 4%. Although these errors are relatively small, it is possible that they may be propagated during the inversion process and result in distorted resistivity models. If the true positions of the electrodes along a curved/angled survey line can be measured, the error in the calculated apparent resistivity values may be compensated for by calculating the true geometric factors. Corrections to the pseudo-depths may be made by calculating the true median depth for each measurement position. The lateral displacement of the measurement position is a consequence of the survey geometry and cannot be compensated for.
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Model Studies of the Propagation of Errors During the Inversion of 2D Resistivity Data Recorded with the Wenner (α) Geometry Along Curved and Angled Survey Lines
More LessWhen recording 2D resistivity data along curved or angled survey lines, the recorded apparent resistivity data will be affected by the fact that the true geometric factors will differ from the assumed geometric factors of a straight survey line. Numerical model studies show that, although the errors in the apparent resistivities may be small even for large angles and curvatures, these errors may rapidly increase in magnitude during inversion. This is particularly true during the inversion of data recorded along angled survey lines where input errors may increase by an order of magnitude within a couple of iterations. Although it seems that angles should be avoided in favour of curves when it is not possible to perform a resistivity survey along a straight line, the apparent resistivity data recorded along an angled survey line may very readily be corrected prior to inversion if the true electrode positions can be measured.
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Inversion of Gravity and Magnetic Gradient Data
Authors Peter K. Fullagar and Glenn A. PearsGradiometry has come of age in the past decade, with the development of airborne systems capable of measuring full gravity and magnetic gradient tensors. Airborne gravity gradiometry in particular represents a stunning technical achievement, delivering a completely new capability to exploration. Gravity gradients can now be acquired rapidly in remote and inaccessible areas. Airborne magnetic gradiometry has been available for many years, albeit for a limited number of components. Full tensor magnetic gradiometry is a very recent development. There are several technical advantages of gradiometry, including interpolation between lines and, for magnetics,suppression of diurnal effects, potential for characterisation of remanence, and (for the theoretically minded) the satisfaction of dealing with true potential fields. In terms of interpretation, gradiometry offers greater sensitivity to the near-surface than conventional gravity or magnetics. This is a blessing for resolution of shallow structure and stratigraphy, and for definition of outcropping or sub-cropping targets, e.g. kimberlites. However, enhancement of shallow features can be a disadvantage in exploration for buried targets. The problem is compounded by the inherent variability of the shallow sub-surface, owing to weathering and to transported cover. Finally, sensitivity to the near-surface also implies sensitivity to topography and flying height. This paper explores these issues and others arising during geologically-constrained inversion of gradiometer data. Analysis of gradiometer data alone is not sufficient to fully prescribe the sub-surface distribution of density or susceptibility: acquisition of full tensor data does not deliver us from non-uniqueness. Geological and petrophysical information, primarily derived from drill holes, is required to reduce uncertainty in interpretation. In order to capture both geology and physical properties, a model must be both categorical and quantitative. Such ‘geo-physical’ models are a force for integration in their own right, and also offer a number of practical advantages over pure property models. In particular, geological boundaries can be recognised, and therefore modified (subject to drill hole pierce point constraints); thus, when applied to a geological model, inversion can adjust its geometry as well as its density or susceptibility distribution.
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Early Holocene Extensional Tectonics in the South-Eastern Cape Fold Belt, South Africa
Authors M.L. Goedhart and P.W.K. BoothThe current seismo-tectonic model for South Africa depicts the south-eastern Cape Fold Belt as a tectonically stable intraplate-type environment, where earthquakes are relatively infrequent to rare, particularly large surface-rupturing events. The closest surface rupture recorded by the SANSN, which underpins the model, is the extensional Mozambique 7.2M event on 22 February 2006, related to southward propagation of the East African Rift. A similar 84km-long extensional surface rupture has been found along the Kango fault within the southern Cape Fold Belt, 50km east of Oudtshoorn. Optically stimulated luminescence dating indicates it occurred around 10,620 ± 509, at the start of the Holocene. This fault, together with the adjoining Baviaanskloof and Coega faults, and other nearby adjacent faults, is the subject of a palaeoseismic investigation to determine the Quaternary tectonic history of the south-eastern continental margin. This region of South Africa has little seismic information, and the record is too short to include the long recurrence intervals of large, surface damaging earthquakes typical of intraplate regions. The data reported includes the location and extent of the surface rupture, the local stress direction, the date and magnitude of the most recent event, the minimum recurrence interval, and maximum slip rate. The earthquake environmental effects are also evaluated according to the new Environmental Seismic Intensity Scale (ESI 2007), to provide an independent assessment of seismic intensity. A potential local tectonic driver, the Cape Isostatic Anomaly (CIA) is also discussed, as well as a possible trigger mechanism leading to the rupture. Results should facilitate PSHA of several new critical facilities planned for the province.
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Flooding-induced Seismicity in Mines
More LessMany South African gold mines will flood when they close, as the groundwater will gradually fill the mining voids. Preliminary investigations have shown that flooding of mines can generate increased levels of seismicity. The purpose of this paper is to create awareness that fluid-induced seismicity will become increasingly important in South Africa when closed mines are allowed to flood. Such flooding-induced seismicity can have significant environmental and socio-economic consequences, and may endanger neighbouring mines and surface communities. Even though fluid-induced seismicity has often been observed in non-mining settings (e.g. filling of dams, oil-well stimulation and hydrothermal fields), no detailed study of seismicity associated with flooding of deep mines has ever been conducted anywhere. It is possible that mine flooding could lead to potentially disastrous seismicity, which may result in high continuous pumping costs by the State to prevent or to contain flooding. The potential relationships between flooding and the magnitude and frequency of triggered and induced seismicity resulting from mine flooding need to be researched. A thorough understanding of the interaction between flooding and seismicity will allow the impact of mine flooding on safety to be determined. In particular, the maximum earthquake size resulting from the flooding of deep gold mines in South Africa needs to be determined. The identified risks will in turn allow appropriate mitigating strategies to be developed. Such strategies will influence South African mine closure policies.
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Anisotropy of Southern African Lithosphere and Asthenosphere
Authors Mark P. Hamilton, Alan G. Jones and The SAMTEX TeamObservations of anisotropy, when understood in terms of deformation processes, are critical to illuminating the dynamics of past and present tectonic processes. In particular anisotropy can infer how continents formed, stabilized and interacted with underlying mantle regions in the past, and how they do so today. Seismology and electromagnetic observations of anisotropy are essential if we are to understand the tectonic history of a region. Seismic anisotropy, defined from SKS arrivals, is poorly constrained in depth, whereas electrical anisotropy has inherent depth localization but lower spatial resolution. Given the limitations of both sub-disciplines a more robust characterization of anisotropy is achieved by integrating complementary datasets. Southern Africa has now two rich geophysical databases from the SASE and SAMTEX experiments that can be explored, compared and contrasted for lithospheric anisotropy. Doing so suggests a new model to explain SKS observations which is based on plausible tectonic history. The new model combines the “Silver” lithospheric anisotropy and “Vinnik” asthenospheric anisotropy models, and incorporates their differentiation using electrical anisotropy.
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Orthogonal AMS and SPO Fabrics in the MORB-like Rooi-Rand Dyke Swarm of South Africa and Swaziland
Authors Warwick Hastie, C. Aubourg and M. K. WatkeysThe cause of the ca. 185-175 Ma Karoo volcanism in southern Africa has been ascribed to the presence of a mantle plume centred on the Nuanetsi (now Mwenezi) Igneous Province, southern Zimbabwe. In the mantle plume model, this area is considered to represent a triple junction between the WNW-trending Okavango Dyke Swarm (ODS), the ENE-trending Sabi monocline and the N-S trending Lebombo monocline. The plume model predicts that magma flow in Karoo dykes of the Lebombo monocline should be away from the plume head and should be sub-horizontal in the distal regions. A brief study of the anisotropy of magnetic susceptibility (AMS) of 23 dykes in the MORB-like Rooi-Rand dyke swarm (RRDS) is presented. The AMS in the samples results from fine-grained, Ti-poor magnetite which in 20 dykes defines fabric sub-parallel to the dyke plane, consistent with the plume model. The magnetite defines a weakly anisotropic and dominantly oblate fabric. From a total of 10 dykes studied for plagioclase mineral shape preferred orientation (SPO), 8 have a dyke-parallel foliation most consistent with vertical magma flow. The plagioclase grains define a weakly anisotropic, oblate fabric, which is magmatic in origin. In 8 dykes this fabric is coaxial with the AMS fabric. However, in 40% of the dykes, the fabric defined by the SPO of opaque grains is non-coaxial with AMS and is at a high angle to the dyke plane and dips steeply. The non-coaxial AMS and SPO fabric, coupled with the orthogonal SPO fabrics suggests that late-stage lateral flow of relatively high viscosity magma has occurred. This results in a fabric which most workers would regard as “inverse” and/or non-magmatic, and, therefore, would misinterpret.
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Geotherms, Lithosphere Thickness and Sedimentary Basins
By Chris HattonA relation between the thickness of the lithosphere and geothermal gradients in the mantle can be extracted from two independent sets of observations. One set is the relation between lithosphere thickness determined from magnetotelluric measurements and surface heat flow. The other set of observations is the temperature-depth array that can be extracted from xenoliths in kimberlites. Both sets lead to the conclusion that an increase in the temperature at the Moho is accompanied by thinning of the lithosphere when the lithosphere is thinner than 180 km, but is accompanied by thickening of the lithosphere when the lithosphere is thicker than 200 km. The corollary of this conclusion is that the same high Moho temperature can be associated with very thin or very thick lithosphere. Switching from very thin to very thick lithosphere can initiate the formation of a sedimentary basin. Continued increase in Moho temperature results in further thickening of the lithosphere and growth of the sedimentary basin.
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A Lithospheric and Temperature Study from the South Atlantic
Authors Katja K. Hirsch, M. Scheck-Wenderoth, S. Fishwick, Y. Maystrenko and J. SippelThe East African continental margin is a passive volcanic margin that experienced a long post-rifting history after break up in Early Cretaceous times. The break up resulted in the formation of a number of basins along the margin. The by far largest depocentre in the South Atlantic, the Orange Basin, is the location of the here represented study. The main goal is to combine information on the geometry and properties of the sedimentary part of the system with data on the geometry and physical properties of the deep crust. It was also aimed to integrate both the continental and the oceanic parts of the margin into a consistent 3D structural model on a lithospheric scale to study how far these features give insights into the deeper lithospheric mantle structure. We compare the depth of the Lithosphere-Asthenosphere-Boundary calculated obeying the principles of Isostasy to results based on surface wave tomography and their influence on the thermal field of the margin. A 3D temperature model was evaluated for the passive continental margin of the South Atlantic including the lithospheric structure of the margin. We evaluate a case study for different scenarios to estimate the influence of sediments and crustal structures on the thermal field. The calculated conductive field is constrained by temperature measurements and 3D gravity modelling. At the Norwegian continental margin it has been found that a differentiation of the physical properties of the lower crust and the mantle is needed between the oceanic and continental domains to explain the observations. We aim to compare the younger setting of the Norwegian continental margin with the old passive margin in the South Atlantic. In particular, the South Atlantic is interesting since the southern half of the continent is elevated and the question arises how this might be linked to a deep seated thermal anomaly.
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Exploring to Depth in the Shadow of Headframes
More LessBrownfield (near-mine) mineral exploration activity is at a record high – driven by the desire to mitigate risk in these uncertain economic times and the old adage that "the best place to explore is in the shadow of a headframe.” Many companies have purchased “old” mines to gain access to mineralization that was “missed” with previous generations of geoscience technologies and to assess new targets. Today, new deep geophysical technologies are assisting in exploration, ore delineation and ground sterilization.
Historically, however, it has been difficult to apply geophysical techniques around mines. Cultural noise, scheduling, electrical noise, remoteness and resistance to new technologies are some of the traditional obstacles (to performing geophysical surveys in brownfield areas) that have been overcome. One of the new technologies that has proven itself is deep electrical imaging -- made available thanks to the distributed acquisition system (DAS) technology. DAS technologies are characterized by a large multi-channel, fixed receiver array and several other factors that together contribute to improved depth of penetration, data quality and detectability. In this paper, we review the components and capabilities of DAS, focusing on one system in particular, for brownfield work, including near-mine and minesite applications. Three case studies, two from porphyry copper environments in western Canada and a gold project from Bulgaria, are presented. These case studies represent the state-of-the art in geophysics for brownfield work and are a novel application for current DAS technologies.
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Gravi-magnetic Anomalies of Uniform thin Polygonal Sheets
Authors Horst Holstein, Des Fitzgerald and Costas AnastasiadesThin planar sheets are useful gravitational and magnetic models of dykes and veins treated as two-dimensional limit cannot be reached this way on account of the floating point finite precision. We derive the analytical zero thickness for the gravity potential while maintaining finite total mass. We use the concept of gravi-magnetic similarity to extend the thin-sheet potential formula to include the potential, field and field gradient in both gravity and megnetic cases, thereby generalising other studies that have obtained isolated polygonal thin-sheet anomaly solutions. We compare the anomalies computed by the new formulae to those of corresponding finite thickness targets, and to the finite differences estimates of the field and field gradient obtained from numerically differentiated thin-sheet potentials. In both cases a second order rate of approach to the limit is observed, verifying the correctness of the new formulae. Thin-sheet solutions are attractive for their reduced computational burden compared to full parallelepiped solutions, while the stacking of thin sheets may be used to stimulate variable density or magnetization targets. It is anticipated than thin-sheet solutions presented here will find application in gravi-magnetic modelling.
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Shale Gas: an Unconventional Hydrocarbon Resource in South Africa? Some Preliminary Observations
Authors Brian Horsfield, Hans-Martin Schulz and Maarten de WitGas shales are currently amongst the hottest plays in the United States as a result of high gas prices, and the remarkable technological successes exploiting the Barnett Shale of the Fort Worth Basin. Furthermore, viable gas shales most likely occur in many developed basins where an underutilized distribution infrastructure exists and markets are readily accessible. Annual natural gas production from shale gas reservoirs in the US is approximately 1.0 Tcf and comes from more than 40,000 shale gas wells (around 6-10% of total natural-gas production in the US today; numbers vary according to reference). While high energy prices and advances in fracturing technology have enabled shale gas production to become a lucrative reality, it is the prediction of gas concentration, partition behaviour and rock properties ahead of drilling that is of paramount importance for reducing risk and identifying “sweet spots” or fairways. In other words, there is still a lot to learn scientifically about how shale gas systems actually work. Significant advances have been made over the last five years, but there are great opportunities for major advances in science and technology. No commercial shale-gas enterprises are currently known from outside of North America. Estimations of more than 16,000 Tcf worldwide shale-gas resources are indications that a tremendous potential exists for future growth. Approximately 500 Tcf is forecast for Europe, and targets in Europe include the Posidonia Shale (Jurassic), Alum Shale (Cambrian), Wealden (Cretaceous), Mikulov Formation (Jurassic) and Riga Formation (Silurian). About 200 Tcf shale gas potential has been forecast for sub-Saharan Africa. As far as South Africa is concerned, and while numerous black shale-containing formations occur throughout the stratigraphic column, the Whitehill and Prince Albert Formations and its equivalents (Permian) stand out as perhaps the prime candidate for exploitation. It consists of dark, carbonaceous lacustrine shale overlain in part by siliciclastic turbidites and tuffs, and was deposited in a sea-level highstand under strongly anoxic redox conditions (Faure and Cole, 1999; Summons et al., 2008). While its organic richness is beyond doubt, thermal maturity will likely play the critical role in distinguishing gas shale versus oil shale potentials. By analogy with the Barnett Shale and Marcellus Shale plays of the United States, advanced thermal maturity levels will be prerequisite in order to achieve high gas-in-place and enable high gas flow rates to be maintained. The role played by the Karoo basalts in defining maturation history is of paramount importance. Extremely high maturity levels (Rm = 2-4%) have already been documented in cores of the Whitehill Formation, and lateral continuity demonstrated by MT surveys (Branch et al., 2007 Inkaba yeAfrica Special Volume). The current contribution will outline the attributes of gas shales in the United States, and explore the potential targets in South Africa, most notably the Whitehill and Prince Albert Formations.
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Wavelet-based Semblance Analysis Applied to Geophysical Borehole Data
Authors Celeste Johnson, Susan Webb, Gordon Cooper and Fabio FrescuraWavelet-based semblance analysis is a relatively new measure which allows for the similarity investigation of local phase relationships between two data sets as a function of frequency (or wavenumber) and time (or distance). The potential for applying this method to geophysical datasets is explored, with quantitative results for two cases. The first case is where two types of data come from the same section of a borehole, and the second case where data comes from similar stratigraphic locations but from different boreholes. Source data comprises density and magnetic susceptibility data from the Bellevue (BV-1) and Moordkopje (MO-1) boreholes located within the Northern Lobe of the Bushveld Complex.
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Fresh Geoscientific Data from the Natal Valley and Mozambique Ridge
Authors Wilfried Jokat, V. Leinweber, K. Kopsch and M.K. WatkeysAlthough the broad break-up sequence of Gondwana around southern Africa during the Jurassic and Cretaceous is understood, the refit of Africa and Antarctica remains unclear. This is due to uncertainty regarding the areas both onshore and offshore of SE Africa that are underlain by oceanic crust as well as the ages of that oceanic crust. An example is the enigmatic Mozambique Ridge that has been variously proposed to be either continental or oceanic in origin (or both). In order to help resolve these issues, during 2009 a marine geophysical survey was undertaken over the Natal Valley and the Mozambique Ridge. This cruise collected magnetic and gravity data, together with swath bathymetry and echo-sounder data. The results revealed that the magnetic anomalies of the southern Natal Valley are consistent with earlier work whereas the Northern Natal Valley is more complex than previously suspected. The Mozambique Ridge contains striped magnetic anomalies indicative of the ridge having been formed at a mid-ocean ridge.
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The SAMTEX Experiment: Overview and Preliminary Results
The Kaapvaal Craton is one of the world’s best natural laboratories for studying the lithospheric mantle given the wealth of xenolith and seismic data that exist for it. The Southern African Magnetotelluric Experiment (SAMTEX) was launched to complement these databases and provide further constraints on physical parameters and conditions by obtaining information about electrical conductivity structures within the lithosphere. Initially, magnetotelluric data acquisition was planned on profiles spatially coincident with the Kaapvaal Seismic Experiment. However with seven more partners joining the original four through the course of the experiment, SAMTEX was enlarged from two to four phases of acquisition, and extended northwards to cover much of Botswana and Namibia. The complete SAMTEX dataset now comprises MT data from over 730 distinct locations in an area of over one million square kilometres, making SAMTEX the largest regional-scale MT experiment conducted to date. Preliminary images of electrical resistivity and electrical resistivity anisotropy at 100 km and 200 km, constructed through approximate one-dimensional methods, map resistive regions spatially correlated with the Kaapvaal, Zimbabwe and Angola Cratons, and more conductive regions spatially associated with the neighbouring mobile belts and the Rehoboth Terrane. Known diamondiferous kimberlites occur primarily on the boundaries between the resistive or isotropic regions and conductive or anisotropic regions.
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Correlation of Lithospheric Velocity and Electrical Conductivity for Southern Africa
Authors Alan G. Jones, Stewart Fishwick, Rob L. Evans and The SAMTEX TeamSouthern Africa is the world’s premier location for studying the Sub-Continental Lithospheric Mantle (SCLM) given the abundance of geophysical and geochemical data that now exist for it. In particular, the Southern African Seismic Experiment (SASE) and the Southern African Magnetotelluric Experiment (SAMTEX) have added a wealth of seismological and electrical data that can be explored for physical property information and compared and contrasted both qualitatively and quantitatively. Qualitatively there is significant spatial correlation between low velocity and low resistivity regions and between high velocity and high resistivity regions. Adopting a quadratic relationship between shear-wave velocity and resistivity, based on mineral physics arguments, and predicting the velocity from the observed resistivity shows that the two are compatible except for two distinct regions. This relationship requires that resistivity be controlled by bulk property effects, particularly temperature variation, and not by minor conductive phases.
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SAMTEX – Quo Vadimus?
More LessThe SAMTEX project is now in its interpretation phase after completing four large phases of acquisition. Broadband and long period magnetotelluric data have been acquired at over 730 locations in an area in excess of one million square kilometres, making it the largest survey of its kind to date. Coverage by SAMTEX is of almost all of Botswana, most of Namibia, and much of South Africa. SAMTEX has demonstrated the utility of regional-scale magnetotellurics for lithospheric imaging, so the natural question to ask is “Where next?” This presentation is one with audience participation to define outstanding problems in Southern and Central African tectonics that can be addressed using this unique geological mapping tool.
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Source Mechanisms of Mining-related Seismic Events in the Far West Rand, South Africa
Authors B.B. Kassa, J. Julià, A.A. Nyblade and R.J. DurrheimThe Far West Rand mining district is known for its deep gold mines and the associated high levels of seismic activity. Thousands of tremors are recorded every day by geophones installed in the mine workings. These events range in local magnitude ML from -1 to 4. Some of the biggest events (ML>3) are external to the network deployed on a single mine. We combined data from three mines (Savuka, Tautona and Mponeng) to improve the location of these events. Source mechanisms were calculated by minimizing the L2 norm of the difference between the observed and predicted P, SV and SH spectral amplitudes, with visually assigned polarities. The preliminary results from the Savuka mine show that the major principal stress is compressional, oriented near to the vertical, and with a significant isotropic component in the moment tensor solution.
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Magnetotelluric Imaging Across a Neoproterozoic Collision Zone: Damara Belt and Surrounding Tectonic Blocks
The Southern African Magnetotelluric Experiment (SAMTEX) is the largest ever land-based magnetotelluric (MT) project. The main objective of the project is to define the geo-electric structure across the region in order to gain a better understanding of Archean and Proterozoic tectonic processes. Only the MT profiles crossing the Rehoboth Terrane, the Neoproterozoic Ghanzi-Chobe/Damara belts (collectively termed the DMB) and the southern Angola craton are the focus of this study. One of the ways in which geo-electrical structural information is obtained is by detailed analysis of the measured impedance tensor. The Groom and Bailey decomposition technique was applied to the MT data and indicates significant depth and along-profile variations in geo-electric strike and dimensionality on all transects crossing these three tectonic units (i.e. Rehoboth Terrane, Angola craton and the DMB). The geo-electric strikes are generally parallel to the north-east trending tectonic fabric as inferred from the magnetic data, but the significant strike variations with depth are expressions of heterogeneity in the lithospheric structure. The Rehoboth terrane, south of the DMB, exhibits a strongly one dimensional (1D) to moderate two dimensional (2D) structure, with preferred strike directions in the range 200-450 20for the crust-mantle period (i.e. depth) range, indicating little crust-mantle decoupling. The DMB appears to be moderately 2D at lower crustal and upper mantle depths (10-100 s) with no consistent/preferred strike direction and significant phase differences between the conductive and resistive directions. North of the DMB and into the Angola craton there are significant variations in geo-electric strike direction and dimensionality at most sites for lower-crustal and upper mantle lithosphere. Our results further indicate that the profiles have to be divided into smaller areas having similar strike directions to allow for 2D modelling and inversion.
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New Developments in Probabilistic Seismic Hazard Analysis: Common Problems and their Resolution
More LessThe purpose of this short paper is to highlight and discuss several frequent problems which one faces in Probabilistic Seismic Hazard Analysis (PSHA). The potential difficulties are:
- how to take into account the incompleteness of seismic event catalogues,
- how to handle the uncertainties within the seismic event catalogues,
- how to take into account the inadequacies inherent in the selected model of seismicity,
- how to delineate seismogenic source zones.
The lecture attempts to address the above problems, to provide tools for their solution and to present a coherent methodology for efficient assessment of PSHA.
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